If a crowed of people lynch someone who they believe is guilty of a crime, they kill him without a legal trial, usually by hanging. (Cambridge Dictionary)
(of a group of people) kill (someone) for an alleged offence without a legal trial, especially by hanging. (Oxford Dictionaries)
If a group of people lynch someone, they kill that person without letting them have a trial, especially by hanging, because they believe that the person has committed a crime. (Collins Dictionary)
What is lynching and why killer mob goes unpunished.
The Meaning of India’s ‘Beef Lynchings’.
The High and Low Tech of It: The Meaning of Lynching and the Death of Emmett Till.